Effective Intraparticle Diffusivity Research Articles

酸化鉄ペレットの還元時における粒子内温度

Intraparticle temperatures of the porous iron-oxide pellets having about 30% porosities during the gaseous reduction were measured by a thermocouple embedded into the center of the sample and the reduction rates of the similar samples were observed by a thermo-gravimetric method. Those intraparticle temperatures reflected that the reduction of iron-oxide was a multi-stage successive reaction and the variations of the temperatures with the progress of reduction corresponded to the reaction rates and heats of each reduction step.To predict theoretically such a temperature variation, the rate equations of the chemical reaction and the heat transfer were developed on the basis of the multi-interface unreacted core model. The calculated variations of the temperature by these equations adopting appropriate rate parameters coincided fairly well with the experimental results.Further, the errors of the rate parameters by ignoring the heat transfer resistances in the analysis of the experimental data of reduction were discussed from the theoretical calculation for the single step reduction. The exact values regarding the chemical reaction rate constant can be obtained but the considerable deviations may arise about the intraparticle effective diffusivity.

DIFFERENCE IN EFFECTIVE DIFFUSIVITY UNDER REACTIVE AND NON-REACTIVE DIFFUSION CONDITIONS

On the basis of a pore model, a difference in average pore radius under reactive and non-reactive diffusion conditions is discussed. This model suggests a difference in intraparticle effective diffusivity value (pore volume diffusivity with no surface diffusion contribution) between these two systems. Experimental pore tortuosity measurements are also reported for commercial catalyst pellets.

Mass diffusion and isothermal catalytic selectivity

An elementary analysis provides a general expression for isothermal selectivity in the first-order reaction sequence A → B → C as catalysed by porous pellets consisting of micropores branched from macropores. Selectivity (yield of B) is concisely related to the intrinsic rate constants, the micro- and macropore effectiveness factors and the fluid phase transport coefficients for the pertinent species. Conditions corresponding to zero and negative selectivity are specified in terms of effective rate coefficient and intraparticle diffusivity ratios for a given intrinsic (surface) selectivity.